Q: Let's say I have 4 MP, we're at onset range, and I want to use the following sequence of actions.1. Make an attack at onset range with 1 MP.2. Make another attack with 1 MP, thus moving to melee range and the attack happens at melee range.3. Spend 1 MP to move back to onset range.4. Spend my last MP to make another attack at onset range.

Is this a legal sequence of moves, and if so what range are we in at the end of it?

A: Yes and you would be at onset range at the end of this sequence.

I see. So essentially the "auto move forward to melee" only happens once per player's turn.

It happens on your second attack from Onset range.

After I "auto move forward" on the second attack and then move back, I don't "auto move forward" again on the third attack.

No because the third attack was your "first" attack done from onset range after getting back into onset by spending your third MP. The first attack you do from onset range leaves you at onset range, it's only the second attack which "auto moves" you into Melee range. This is how a fight with weapons really tends to work for some reason, if you have ever tried it out you probably know what I mean. If you are attacking aggressively, by default you tend to get closer - unless you make an effort to maintain distance.... and that closer range is a dangerous place to be. In Joachim Meyers 16th Century Fechtbuch he reccomends no more than 4 exchanges in "Krieg" range (the middle range) after which he suggests if you haven't defeated your opponent you should pull back out to a safer distance and start over.

Of course whether a safer distance is 'safer' depends on your weapon. If you have a dagger and your opponent has a spear, it's safer for you at close range.

Also, another question: When you get a Free Dice, can you use it even if you don't use any dice from your regular Martial Pool? For example, suppose I have Point Control, and an opponent tries to move into grapple range but I don't have any MP left. Can I still make the AoO with the one free dice, or would I have to have had an MP to spend to make the attack before I could use the free dice?

Normally you have to spend a die to get a Free Dice, though some Feats and some Martial Feats make specific exceptions to this rule, for example the "Bind and Batter" MF.

G.

EDIT: Keep in mind that this default rule where you get the "free move forward" with your second attack is, like everything in the Codex, essentially optional. It was done, as I said, to mimic HEMA. If you prefer to change it to something more symmetrical, for example ruling that any change of range costs 1MP and no change of range happens 'automatically', I don't think that would have any negative effects on gameplay. You might just need to tweak one or two things.